The present invention relates to vacuum-type circuit interrupters, such as are used in electric power transmission and distribution, and switchgear assemblies. In a vacuum-type interrupter, a pair of current carrying contacts are moved apart to effect circuit interruption with an arc being initially struck between the separated contacts and thereafter extinguished. Recent work has been directed towards increasing the operating voltage and current interrupting capability of such vacuum interrupters. It is well known that as the current which is to be interrupted is increased, a current value will eventually be reached at which an anode spot will form, and the rate of electrode erosion during arcing is considerably increased. Such electrode or contact erosion is a significant limiting factor in reliable long-life operation of such interrupters. Such a high current anode spot results in evolution of the contact material and severe localized heating accompanying by a large increase in the arc voltage.
It is now well known that an axial magnetic field directed along the interrupter longitudinal axis parallel to the arc current path will tend to create a diffused arc current condition, minimizing the possibility of destructive anode spot formation. Such axial field vacuum interrupters are described in representative U.S. Pat. No. 4,117,288, and U.S. Pat. No. 4,260,864. In another recent axial field vacuum interrupter teaching in U.S. Pat. No. 4,196,327, the field generating coil disposed behind the main arcing contact has a plurality of current carrying spoke members extending inward from the axial field generating coil. This structure defines current paths which define a multipole magnetic field for these electrodes. It is generally acknowledged that an axial magnetic field is effective to increase the current handling capacity of a vacuum interrupter because it divides the arc current between many cathode spots leading to a diffused arc. For high current carrying capacities, such devices have generally had large electrode diameters, and the magnetic field coils are designed to be effective over the entire electrode area. For devices in which a coil member is disposed within the interrupter envelope, the magnetic field applied in the axial direction varies with the radial distance from the axis of the device, so that a preferred radial position along the arc contact exists at which the arc voltage will be a minimum, and currents at other radial distance from the axial will tend to be extinguished. This had led to the provision of a raised annular ring arcing surface on large diameter arc contacts with the arcing current being concentrated in this annular raised portion leading to relatively inefficient use of the electrode area.
It is generally desired in providing a high current capacity vacuum interrupter contact that the arc current energy be distributed over as large a portion of the contact area as possible to minimize anode spot formation.